1. Field of the Invention
The present invention relates to a CMOS image sensor having a cell array portion in which single cells are arranged in an array and a peripheral circuit portion that drives these cells, wherein the CMOS image sensor has a region for absorbing a noise charge in the substrate around the cell array portion, and a manufacturing method for the same
2. Description of the Related Art
FIG. 5 shows a cross-section of a conventional CMOS image sensor. FIG. 5 shows a cross-sectional schematic diagram of a photodiode in a cell at the edge part of the cell array formed on the P-type silicon substrate 1 and the peripheral circuit portion N-channel MOSFET. The N-channel MOSFET is formed in the P-type well 5 formed on the P-type silicone substrate 1. In the photodiode portion in the cell, the P-type well 5 is not formed, while a deep P-type well 6 is formed at a deeper position, and optical barrier layer 7 is formed with openings above the photodiode portion. Thereby, a high sensitivity is attained with respect to incident light up to near infrared light.
An N-type diffusion layer 2 is formed on the substrate surface and then a P-type diffusion layer 8 is formed on this surface to comprise a buried photodiode structure. Thereby, an improvement of image quality is attained by suppressing noise generated in the vicinity of the surface. Between the photodiode in the cell and the peripheral circuit portion N-channel MOSFET, a field oxide film 11 is formed to separate these elements.
However, in this type of CMOS image sensor, there is the problem that the OB (optical black) cells, which determine the darkness level, and the effective cells are influenced by the noise charge that is generated on the periphery thereof and strays over the substrate to cause a deterioration of the image quality.
What produces this kind of problem is that the noise charge that is generated at the periphery circuit portion and strays over the substrate is taken in as noise in the photodiode of the effective cells and OB cells in the cell array, and that the charge that that is generated in the effective cell portion in the cell array and moves over the substrate to the OB cell is taken in as noise in the photodiode of the OB cells.
In consideration of the above-described problems, it is an object of the present invention to provide a CMOS image sensor that can decrease the influence of the noise charge on the OB cells that determine the darkness level, and can prevent deterioration of the image quality.
In addition, it is an object of the present invention to provide a method of manufacturing a CMOS image sensor that can decrease the influence of the noise charge on the OB cells that determine the darkness level, and can prevent deterioration of the image quality without adding manufacturing processes.
In order to solve the above-described problem, in a first aspect of the present invention, in a CMOS image sensor having a cell array portion in which single cells are disposed in an array and a peripheral circuit portion that controls this cell array, a noise charge absorption region is provided on the periphery of the cell or the cell array, a second conductive diffusion layer that is formed on a first conductive silicon substrate surface is included in the noise charge absorption region, and the second conductive diffusion layer is connected to a constant potential.
In a second aspect of the present invention, the invention according to the first aspect is characterized in that a noise charge absorption region is formed between the cell array portion and this peripheral circuit portion.
In a third aspect of the present invention, the invention according to the second aspect is characterized in that a noise charge absorption region is formed so as to enclose the periphery of the cell array portion.
In a fourth aspect of the present invention according to the first aspect, a CMOS image sensor that has an effective cell group having a cell array portion in which single cells are disposed in an array and collects the signal charge in the cell array and an OB cell group that is in proximity to the effective cell group and detects the darkness level is characterized in that a noise charge absorption region is formed between the effective cell group and the OB cell group.
In a fifth aspect of the present invention, the invention according to the first aspect is characterized in that a noise charge absorption region is formed between effective cells in the cell array portion.
In a sixth aspect of the invention, a manufacturing method for a CMOS image sensor having a cell array portion in which single cells are disposed in an array and a peripheral circuit portion that controls this cell array is characterized in that a PN junction that forms the photodiodes of the effective cells and the OB cells and a PN junction in the noise charge absorption region are formed simultaneously.